Uroscopy?

It actually makes sense. Urine is produced by the body and therefore should contain information about that body.
Medieval medicine is generally characterized as either hocus-pocus or butchery. While it is true that doctors could do little in the area of diagnostics and treatments, they did the best with what they had. There were complex uroscopy charts that associated diseases with various urine colors (not all of them were wrong) and it was through one aspect of urine examination, tasting, that diabetes was discovered. We still use urinalysis today, and I was taught to do my own, which I can still do. Tasting not required.